1. Field of the Invention
This invention relates to a narrow band acoustic source and to a method for constructing and using said source.
2. Description of the Art
Acoustic sources have traditionally been used to obtain well logs with good resolution of geologic features. A source is lowered into a well bore and an acoustic signal is generated, this signal usually having a frequency in the range of 5 kHz to 25 kHz. The signal is then received at receivers which are typically part of the same downhole sonde as the source. These signals provide vertical resolution of features on the order of 1 foot (0.3 m). This technique gives excellent information about the area immediately adjacent to the well bore; however, conventional sources penetrate only a few feet (on the order of 3 meters) beyond the wall of the well bore. The vertical range (distance from sonne to receiver) of the signals is of only a few tens of feet. Well logging with such sources thus does not provide information about the formation very far from the wellbore
The limited penetration of conventional acoustic sources is attributable to several factors. Travel through the earth more severely attenuates high frequency signals such as those which are produced by conventional acoustic devices for well logging. Sources currently used for seismic surveying, in which the signal must travel distances of 1000 feet (305 m) or greater, typically operate in the range of approximately 100 Hz and do not have good vertical resolution. Shallow penetration of conventional high frequency acoustic sources is also explained by the relationship of the total power of the device, P.sub..GAMMA., to the amplitude A of the signal at the central frequency, A (.omega..sub.o) and the bandwidth of the signal, .GAMMA.(Hz). As shown in Equation (1): EQU P.sub..GAMMA. =2.pi..GAMMA..vertline.A(.omega..sub.o).vertline..sup.2Eq. ( 1)
The total power, P.sub..GAMMA., is generally fixed by the instrumentation and power supply system. Therefore, is bandwidth decreases, amplitude increases. Total power as also limited by rapid triggering of the source. In typical logging operations, the source is triggered approximately 10-20 times per second. This rate results in less energy being available per pulse, leading to a further amplitude decrease.
A source with narrow bandwidth and a slower cycle rate would generate a more powerful signal and be very desirable.